For 2021, the $300 ($600 MFJ) "above the line" charity deduction is after AGI (in 2020 it was before AGI). It now goes on line 12b of form 1040 (in 2020, it was line 10b).
Itemized deductions (including charity) on schedule A, as well as the standard deduction, has always been after AGI.
So for 2020, a taxpayer with $50,000 of charitable deductions could deduct $300 to reduce AGI, but the other $49,700 would go on schedule A and not affect AGI (but would reduce taxable income). For 2021, the entire $50K deduction comes after AGI.
